The present invention relates in general to systems for performing photoplethysmographic measurements of various blood analytes and other hemodynamic parameters.
In the science of photoplethysmography, light is used to illuminate or trans-illuminate tissue for the purpose of measuring blood analytes or other hemodynamic or tissue properties. In this monitoring modality, light is injected into living tissue and the light which is not absorbed by the tissues is detected a short distance from the entry point. The detected light is converted into an electronic signal, indicative of the received light signal from the tissue. This electronic signal is then used to calculate one or more physiologic parameters such as arterial blood oxygen saturation, heart rate, cardiac output, or tissue perfusion. Other blood analytes that may be measured by photoplethysmography include the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood.
The first commercial use of photoplethysmography in medicine was in the pulse oximeter, a device designed to measure arterial blood oxygen saturation. Since the inception of this device, this monitoring modality has been used to detect more and more different parameters. For example, a device has recently been disclosed which is capable of measuring the percentages of four different analytes in the arterial blood, including oxyhemoglobin, carboxyhemoglobin, methemoglobin and reduced hemoglobin. To make these measurements a number of different bands of light must be used, with each light band possessing a unique spectral content. Each spectral band is usually referred to by the center wavelength, or sometimes by the peak wavelength, for the given band. In the case of pulse oximetry for instance, two different light emitting diodes (LEDs) are typically used to generate the sensing light, the first with a center or peak wavelength around 660 nanometers (nm) and a second with a center or peak wavelength around 940 nm.
As the number of different parameters measured by photoplethysmography increases, so too does the number of different bands of light required to make the measurements. Further, because a fairly high intensity of light, over a fairly narrow spectral range is needed for these measurements, it has been found that the most successful sources of light for these measurements have been discrete, narrow-band emitters such as LEDs or laser diodes. These types of light sources are typically used because broadband sources have too little energy over the desired narrow spectral ranges to provide sufficient signal amplitude for photoplethysmographic measurements.
As the number of light sources, or emitters, used in a single device increases, the problems associated with how to deliver the light to the tissue-under-test also increases. These problems are created by several different factors. These include the ergonomic constraints placed on photoplethysmographic instrumentation by the medical community, cost constraints, reliability concerns, emitter operating parameters, and technical feasibility considerations. In the design of a multi-parameter photoplethysmographic device, it is desirable to use a selection of emitters with the best optical properties possible for the analyte measurements of interest and in an opto-mechanical configuration that best meets the technical, ergonomic, and cost constraints of the instrument.
In the science of photoplethysmography, light is used to illuminate, or trans-illuminate, tissue for the purpose of measuring blood analytes or hemodynamic properties or parameters. In making these measurements it can become necessary to use light from a number of different sources including but not limited to LEDs, incandescent bulbs, or lasers.
Each of these different sources has different properties that dictate how the light will be delivered to the tissue under test. For example, use of LEDs allows the light source to be placed in the sensor and directly proximate to the tissue-under-test. Additionally, it is difficult to couple LED light into a secondary light-delivering apparatus such as a fiber optic cable, making it technically difficult to position the LEDs at a distance from the sensor. Note that for the purposes of this document the xe2x80x98sensorxe2x80x99 is defined as that portion of the system that is placed directly on the tissue-under-test.
By comparison, laser light sources (particularly currently-available conventional semiconductor lasers) require thermal stabilization to maintain a proper spectral output and, when mounted with their necessary thermal controllers and heat sinks, are too bulky to be placed directly on the sensor. Instead the laser or lasers are preferably placed in the instrument or in a housing at some intermediate point on the patient cable. The laser light can be transmitted to the sensor via some type of light pipe, typically a fiber optic cable of some type. Laser light sources are also typically more expensive and considerably more fragile than LEDs, which are additional reasons to place these types of components at a distance from the sensor and in a housing that can adequately protect them.
Incandescent light sources are also difficult to position at the sensor. They tend to be large, hot, and require a certain amount of optics to make them useful for photoplethysmographic devices. Again, these properties make it desirable to place any incandescent sources some distance from the sensor.
With the increasing complexity of photoplethysmographic devices detecting and measuring more and more parameters, it becomes necessary to design systems with large numbers of emitters. In an effort to maximize measurement accuracy, it can be beneficial to utilize a multitude of emitters of different types. For example in the specific instrument of this invention, one capable of monitoring four different species of hemoglobin and the heart rate, it is desirable to use three or more laser diodes and one or two LEDs. This allows for the measurement system to utilize the different optical properties of the different emitters in the same system. For the specific instrument described herein, the optical stability of the LEDs"" output light is required for measurement of low perfusion patients, and the extremely narrow-band output of the laser diodes makes possible the measurement of optical absorption of a hemoglobin species on very steep portions of its extinction curve.
A hybrid optical system, containing multiple emitters of more than one type, makes photoplethysmographic measurements possible that are otherwise not feasible or which could otherwise not be made to clinically acceptable levels of accuracy and precision.
One aspect of this invention therefore is the use of multiple types of emitters in the same photoplethysmographic system. A second aspect of this invention is the optimal placement of the different emitters, with some type or types typically positioned at the sensor and other types positioned in an intermediate housing or inside the instrument housing, to create the most optimal configuration both mechanically and optically while maximizing reliability and minimizing product cost.
This emitter layout creates a problem for the instrument designer in that the lasers are difficult to position at the sensor and the LEDs are difficult to position at a distance from the sensor. In past designs of photoplethysmographic devices, the emitters were all of one type and were all positioned either at the sensor with their output light directly incident on the tissue-under-test or the emitters were all positioned at a distance from the sensor and the light piped to the sensor by a light guide. This invention allows the photoplethysmographic instrument designer to utilize the beneficial optical properties of multiple and different types of emitters in the same design while permitting the positioning of these emitters at different locations in the photoplethysmographic system for optimal coupling of the output light into the tissue-under-test.
A further aspect of this invention is the co-location of the output light from the various sources to cause the light from all emitters to be incident on the same small area of the tissue-under-test, thus forcing the output light from all the light sources to follow the same optical path. This is a necessary condition for accurate photoplethysmographic measurements in tissue.